The present invention relates to methods and apparatus for using a formation tester to perform a pretest on a subterranean formation through a wellbore to acquire pressure versus time response data in order to calculate formation pressure and permeability. More particularly, the present invention relates to improved methods and apparatus for performing the drawdown cycle of a pretest in a formation having low permeability.
Due to the high costs associated with drilling and producing hydrocarbon wells, optimizing the performance of wells has become very important. The acquisition of accurate data from the wellbore is critical to the optimization of the completion, production and/or rework of hydrocarbon wells. This wellbore data can be used to determine the location and quality of hydrocarbon reserves, whether the reserves can be produced through the wellbore, and for well control during drilling operations.
Well logging is a means of gathering data from subsurface formations by suspending measuring instruments within a wellbore and raising or lowering the instruments while measurements are made along the length of the wellbore. For example, data may be collected by lowering a measuring instrument into the wellbore using wireline logging, logging-while-drilling (LWD), or measurement-while-drilling (MWD) equipment. In wireline logging operations, the drill string is removed from the wellbore and measurement tools are lowered into the wellbore using a heavy cable that includes wires for providing power and control from the surface. In LWD and MWD operations, the measurement tools are integrated into the drill string and are ordinarily powered by batteries and controlled by either on-board and/or remote control systems. Regardless of the type of logging equipment used, the measurement tools normally acquire data from multiple depths along the length of the well. This data is processed to provide an informational picture, or log, of the formation, which is then used to, among other things, determine the location and quality of hydrocarbon reserves. One such measurement tool used to evaluate subsurface formations is a formation tester.
To understand the mechanics of formation testing, it is important to first understand how hydrocarbons are stored in subterranean formations. Hydrocarbons are not typically located in large underground pools, but are instead found within very small holes, or pore spaces, within certain types of rock. The ability of a rock formation to allow hydrocarbons to move between the pores, and consequently into a wellbore, is known as permeability. The viscosity of the oil is also an important parameter and the permeability divided by the viscosity is termed “mobility” (k/μ). Similarly, the hydrocarbons contained within these formations are usually under pressure and it is important to determine the magnitude of that pressure in order to safely and efficiently produce the well.
During drilling operations, a wellbore is typically filled with a drilling fluid (“mud”), such as water, or a water-based or oil-based mud. The density of the drilling fluid can be increased by adding special solids that are suspended in the mud. Increasing the density of the drilling fluid increases the hydrostatic pressure that helps maintain the integrity of the wellbore and prevents unwanted formation fluids from entering the wellbore. The drilling fluid is continuously circulated during drilling operations. Over time, as some of the liquid portion of the mud flows into the formation, solids in the mud are deposited on the inner wall of the wellbore to form a mudcake.
The mudcake acts as a membrane between the wellbore, which is filled with drilling fluid, and the hydrocarbon formation. The mudcake also limits the migration of drilling fluids from the area of high hydrostatic pressure in the wellbore to the relatively low-pressure formation. Mudcakes typically range from about 0.25 to 0.5 inch thick, and polymeric mudcakes are often about 0.1 inch thick. On the formation side of the mudcake, the pressure gradually decreases to equalize with the pressure of the surrounding formation.
The structure and operation of a generic formation tester are best explained by referring to FIG. 5. In a typical formation testing operation, a formation tester 500 is lowered on a wireline cable 501 to a desired depth within a wellbore 502. The wellbore 502 is filled with mud 504, and the wall of the wellbore 502 is coated with a mudcake 506. Because the inside of the tool is open to the well, hydrostatic pressure inside and outside the tool are equal. Once the formation tester 500 is at the desired depth, a probe 512 is extended to sealingly engage the wall of the wellbore 502 and the tester flow line 519 is isolated from the wellbore 502 by closing equalizer valve 514.
Formation tester 500 includes a flowline 519 in fluid communication with the formation and a pressure sensor 516 that can monitor the pressure of fluid in flowline 519 over time. From this pressure versus time data, the pressure and permeability of the formation can be determined. Techniques for determining the pressure and permeability of the formation from the pressure versus time data are discussed in U.S. Pat. No. 5,703,286, issued to Proett et al., and incorporated herein by reference for all purposes.
The collection of the pressure versus time data is often performed during a pretest sequence that includes a drawdown cycle and a buildup cycle. To draw fluid into the tester 500, the equalizer valve 514 is closed and the formation tester 500 is set in place by extending a pair of feet 508 and an isolation pad 510 to engage the mudcake 506 on the internal wall of the wellbore 502. Isolation pad 510 seals against the mudcake 506 and around hollow probe 512, which places flowline 519 in fluid communication with the formation. This creates a pathway for formation fluids to flow between the formation 522 and the formation tester 500.
The drawdown cycle is commenced by retracting a pretest piston 518 disposed within a pretest chamber 520 that is in fluid communication with flowline 519. The movement of the pretest piston 518 creates a pressure imbalance between flowline 519 and the formation 522, thereby drawing formation fluid into flowline 519 through probe 512. The drawdown cycle ends, and the buildup cycle begins, when the pretest piston 518 has moved through a set pretest volume, typically 10 cc. During the buildup cycle, formation fluid continues to enter tester 500 and the pressure within flowline 519 increases. Formation fluid enters the tester 500 until the fluid pressure within flowline 519 is equal to the formation pressure or until the pressure differential is insufficient to drive additional fluids into the tester. The pressure within flowline 519 is monitored by pressure sensor 516 during both the drawdown and buildup cycles and the pressure response for a given time is recorded. Formation testing methods and tools are further described in U.S. Pat. Nos. 5,602,334 and 5,644,076, which are hereby incorporated herein by reference for all purposes.
Formation testing tools are ordinarily designed to operate at a single, constant drawdown rate, and the drawdown continues until a set volume is reached. The control systems that determine the drawdown rate, by controlling the movement of pretest piston 518, are often designed to run most efficiently at a fixed drawdown rate. In order to simplify the design and operation of the system, traditional formation testing tools, such as 500, are also designed to draw in a set volume of fluid during each drawdown cycle. A typical drawdown rate is 1.0 cc/sec with a pretest volume of 10 cc.
In normal applications, pretest piston 518 retracts to draw formation fluid into the flowline 519 at a rate faster than the rate at which formation fluid can flow out of the formation. This creates an initial pressure drop within flowline 519. Once the pretest piston 518 stops moving, the pressure in flowline 519 gradually increases during the buildup cycle until the pressure within flowline 519 equalizes with the formation pressure. During this process, a number of pressure measurements can be taken. Drawdown pressure, for example, is the pressure detected while pretest piston 518 is retracting. This pressure is at its lowest when pretest piston 518 stops moving. Buildup pressure is the pressure detected while formation fluid pressure builds up in the flowline. FIG. 2 depicts a typical pressure versus time plot 210 for a constant rate drawdown.
Maintaining a constant drawdown rate can limit the tester's effectiveness in testing low permeability zones, e.g. <1.0 md (millidarcies), because the drawdown pressure can be reduced below the bubble point of the formation fluid, which will cause gas to evolve from the fluid. To achieve a useful pressure-versus-time response from the pretest, once this occurs it is necessary to wait until the gas is reabsorbed into the fluid. The reabsorption of gas into the fluid can take a long period of time, often as much as one hour. This time delay is often unacceptable to operators, and therefore may preclude the collection of pressure-versus-time data, and subsequent calculation of formation pressure and permeability, from low permeability formations.
Another problem encountered when using constant drawdown methods in LWD or MWD applications is lack of available power. In contrast to wireline logging tools that draw their power through the wireline from a source at the surface, in LWD or MWD applications, the measurement tools are powered by batteries and therefore have limited available power. The power used by the system can be expressed by multiplying the change in pressure within the flowline (ΔpFlowline) by the drawdown rate (QDrawdown), or:Power=ΔpFlowline×QDrawdown  Eq.1Therefore, in a low permeability formation where an increased drawdown pressure is required, the power requirements increase for a given drawdown rate. Thus, a large amount of power may be required during the drawdown process, and it may be impractical to provide this power from batteries in a LWD or MWD application.
In order to fully describe the embodiments of the present invention, as well as to illustrate the benefits and improvements of the methods and apparatus, FIG. 1 provides a graphical representation of the operation of a standard formation testing tool, such as the tool of FIG. 5, operating in a low permeability formation. As previously described, the standard formation testing tool 500 is designed to operate with a drawdown rate of 1.0 cc/sec and a pretest volume of 10 cc. In FIG. 1, the low permeability formation from which the sample is collected has a permeability of 0.1 millidarcies (md) or less, and the formation fluid has a bubble point of approximately 700 psi.
FIG. 1 shows plots of pressure versus time, line 102, and drawdown rate versus time, dashed line 104, when attempting to collect a formation fluid sample from a low permeability formation using a conventional constant drawdown rate, such as 1.0 cc/sec for 10 seconds to collect a 10 cc pretest volume. The minimum drawdown pressure, indicated at 110, can drop as much as 10,000 psi below the formation pressure. As mentioned above, in low porosity formations, this minimum pressure 110 can fall below the bubble point 106 of the formation fluid, causing gas bubbles to evolve within the sample. In order to obtain accurate readings, the buildup portion of the cycle must continue until the gas reabsorbs into solution, as at 112, and then sufficient formation fluid is drawn into the tool such that the pressure stabilizes at 114. The gas evolution and reabsorption period, indicated by the portion of the line indicated at 112, takes an extended period of time and this extended period of time is often unacceptable to logging operators. Thus, it is desirable to complete the drawdown cycle without allowing the drawdown pressure to fall below the bubble point of the fluid.
For all of these reasons, it is desired to provide a tool for measuring pressure and permeability without requiring wireline power and without losing effectiveness in low-permeability formations.